Some hearing implants such as Middle Ear Implants (MEI's) and Cochlear Implants (CI's) employ attachment magnets in the implantable part and an external part to hold the external part magnetically in place over the implant. For example, as shown in FIG. 1, a typical cochlear implant system may include an external transmitter housing 101 containing transmitting coils 107 and an external magnet 105. The external magnet 105 has a conventional coin-shape and a north-south magnetic dipole that is perpendicular to the skin of the patient to produce external magnetic field lines M1 as shown. Implanted under the patient's skin is a corresponding receiver assembly 102 having similar receiving coils 108 and an implanted internal magnet 106. The internal magnet 106 also has a coin-shape and a north-south magnetic dipole that is perpendicular to the skin of the patient to produce internal magnetic field lines M2 as shown. The internal receiver housing 102 is surgically implanted and fixed in place within the patient's body. The external transmitter housing 101 is placed in proper position over the skin covering the internal receiver assembly 102 and held in place by interaction between the internal magnetic field lines M2 and the external magnetic field lines M1. Rf signals from the transmitter coils 107 couple data and/or power to the receiving coil 108 which is in communication with an implanted processor module (not shown).
One problem arises when the patient undergoes Magnetic Resonance Imaging (MRI) examination. Interactions occur between the implant magnet and the applied external magnetic field for the MRI. As shown in FIG. 2, the direction of magnetization {right arrow over (m)} of the implant magnet 202 is essentially perpendicular to the skin of the patient. Thus, the external magnetic field {right arrow over (B)} from the MRI may create a torque {right arrow over (T)} on the internal magnet 202, which may displace the internal magnet 202 or the whole implant housing 201 out of proper position. Among other things, this may damage the adjacent tissue in the patient. In addition, the external magnetic field {right arrow over (B)} from the MRI may reduce or remove the magnetization {right arrow over (m)} of the implant magnet 202 so that it may no longer be strong enough to hold the external transmitter housing in proper position. The implant magnet 202 may also cause imaging artifacts in the MRI image, there may be induced voltages in the receiving coil, and hearing artifacts due to the interaction of the external magnetic field {right arrow over (B)} of the MRI with the implanted device. This is especially an issue with MRI field strengths exceeding 1.5 Tesla.
Thus, for existing implant systems with magnet arrangements, it is common to either not permit MRI or at most limit use of MRI to lower field strengths. Other existing solutions include use of surgically removable magnets, spherical implant magnets (e.g. U.S. Pat. No. 7,566,296), and various ring magnet designs (e.g., U.S. Patent Publication 20110022120), all of which are incorporated herein by reference. Among those solutions that do not require surgery to remove the magnet, the spherical magnet design may be the most convenient and safest option for MRI removal even at very high field strengths. But the spherical magnet arrangement requires a relatively large magnet much larger than the thickness of the other components of the implant, thereby increasing the volume occupied by the implant. This in turn can create its own problems. For example, some systems, such as cochlear implants, are implanted between the skin and underlying bone. The “spherical bump” of the magnet housing therefore requires preparing a recess into the underlying bone. This is an additional step during implantation in such applications which can be very challenging or even impossible in case of very young children.
Various complicated arrangements of magnetic elements have been described for use in hearing implant applications. See for example, U.S. Pat. No. 4,549,532; U.S. Pat. No. 7,608,035; U.S. Patent Publication 20110022120; and U.S. Patent Publication 20110264172, which are incorporated herein by reference. However, there is no suggestion that such therapeutic arrangements might potentially have any utility for magnetic attachment applications such as those described above.